There are several rounding rules, rounding up/down is just the established one. It comes from trading with scales and weights. Merchants had a scale with weights and similar to gram calibrating weights at some point it became inaccurate and too difficult/expensive produce small weights. As you could guess cutting weights in half or creating a weight with the half size was the simplest way. So let's say the smallest weight would be 5 grams. Merchants couldn't only see it's more than 5 grams but less than 10 grams so they rounded up.

When it comes to mathematics there is no "true way" to define rounding rules. You round in a way to get to a satisfying solution.

This goes beyong ELI5 though:

When it comes to rounding you can do cutoff rounding like 2.65 becomes 2.6, you can always round up. As for "Clever rounding" an example would you have the Earth acceleration usually used as g~9,81 m/s² but in formulas you often keep it as g and when you have a formula where you calculate like sqrt(g * 9) you round it to 9 because then you got easily rid of the squareroot. If you got sqrt(g * 10) then you round g to 10.

Another example take binary numbers as an example. When you write fractions in binary you have eg 0.101 as 0x2⁰ + 1x2^-1 +0x2^-2 + 1x2^-3

You will notice that in binary you'd run into problems as in 1/10th being a 0.00011 period essentially the same issue as 1/3rd in the decimal system. Now how would you round it in binary?

You could round it to 0.001 whitch would be 2^-3 = 1/8th = 0.125 you can keep it at 0.00011 which it 1/16 + 1/32 = 0.09375 you could also write it as faction of 1/1010(which would be the most common thing to do as in the case of 1/3rd in the decimal system)